The Fish Fauna of Lake Victoria during a Century of Human Induced Perturbations

Frans Witte1,2, Mary A. Kishe-Machumu1,3, Olivia C. Mkumbo4, Jan H. Wanink1,5, Pleun C. Goudswaard1,6, Jacco C. Van Rijssel1,2,7, Martien J.P. van Oijen2

ABSTRACT

Witte, F., Kishe-Machumu, M. A., Mkumbo, O. C., Wanink, J. H., Goudswaard, P. C., Van Rijssel, J.C. & van Oijen, M. J.P. The fish fauna of Lake Victoria during a century of human induced perturbations. In J. Snoeks & A. Getahun (eds), Pro- ceedings of the Fourth International Conference on African Fish and Fisheries, Addis Ababa, Ethiopia, 22-26 September 2008. Tervuren: Royal Museum for Central Africa, ‘Zoological Documentation Online Series’, pp. 49-66. Lake Victoria, by area the largest tropical lake of the world, is well-known for its diverse native fish fauna, which com- prised about 500 endemic haplochromine cichlid species, two tilapiine species and 46 other species belonging to 12 fami- lies. During the past decades, the fish species diversity in the lake has declined dramatically due to human induced per- turbations in the ecosystem. Based on literature and our own research findings we provide an overview of these changes and their most likely causes. During the first half of the last century, the increasing fishing pressure had a great impact on the native tilapiine cichlids and other large fish species. The shift in fish landings showed a classic example of fishing down the food web. Because of the dwindling catches, the Nile perch and four exotic tilapiine cichlids were introduced into the lake in the 1950s. Dramatic changes in the fish fauna occurred in the 1980s, with the upsurge of the introduced Nile perch and Nile tilapia, the decline of wetland zones, and increased eutrophication of the lake. The native tilapiines were replaced by the Nile tilapia, and several of the catfish species showed a dramatic decline, as did the lungfish. Most severely hit were the haplochromine cichlids, which disappeared from large parts of the lake, probably resulting in the extinction of many species. The introduced Nile perch and Nile tilapia, and the native cyprinid Rastrineobola argentea became the dominant fish species. Though water quality deteriorated and fish diversity decreased, fish landings rose from about 100,000 t y-1 in the 1970s to approximately 1 million t y-1 in the period 2005-2007. Since 1999 the biomass of Nile perch declined, whereas that of R. argentea increased. In the same period some of the sublittoral haplochromine species have recovered. Some of the surviving fish species, show remarkable changes in ecological and morphological features relative to the pre-Nile perch period, which seem to be adaptive responses to the changed environment. Although it may be possible to reconcile fisheries sustainability with biodiversity conservation in the lake basin, measures to reduce envi- ronmental stress in the lake are an urgent issue. Keywords: biodiversity, environmental degradation, eutrophication, extinction, fishery, haplochromine cichlids, Nile perch, speciation, species introductions

1 Institute of Biology Leiden, University of Leiden, P.O. Box 9505, 2300 RA Leiden, The Netherlands.

2 Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA Leiden, The Netherlands.

3 Tanzania Fisheries Research Institute, P.O. Box 9750, Dar Es Salaam, Tanzania.

4 Lake Victoria Fisheries Organization, P.O. Box 1625, Jinja, Uganda.

5 Koeman en Bijkerk bv, Ecological Research and Consultancy, P.O. Box 111, 9750 AC Haren, The Netherlands.

6 IMARES Wageningen-UR, Institute for Marine Resources and Ecosystem Studies, P.O. Box 77, 4400 AB, Yerseke, The Netherlands.

7 Jacco C. Van Rijssel is the corresponding author: [email protected] 50 Proceedings of the 4th International Conference on African Fish and Fisheries Lake Victoria’s Fish Fauna during the Past Century – Witte et al. 51

INTRODUCTION Table 1. Total number of fish species native to Lake Victoria arranged by family and number of species that With a surface area of 68,800 km2, Lake Victoria is were introduced into the lake. Sources: Greenwood 1966, 1974; Kaufman & Ochumba 1993; van Oijen 1995; the largest tropical lake in the world. It has a maxi- Seehausen 1996; Witte et al. 2007a. mum depth of 70 m, which is relatively shallow com- pared to other large lakes in East Africa (Fryer & Families/tribes Number Introduced Iles 1972). The lake is well known for its high fish of native species species diversity, dominated by haplochromine cich- species lids (Greenwood 1974; Seehausen 1996; Witte et al. Protopteridae 1 2007a). During the past century, dramatic anthropo- Mormyridae 7 genic changes were observed in the lake’s ecosystem. Alestidae 2 Fishery, species introductions and increasing eutro- Cyprinidae 17 phication had an enormous impact on the fish fauna of the lake (e.g. Ogutu-Ohwayo 1990a; Witte et al. Bagridae 1 1992; Verschuren et al. 2002; Hecky et al. 2010). The Schilbeidae 1 present paper aims to give a review of the changes in Clariidae 6 the fish fauna of Lake Victoria and to discuss the po- Mochokidae 2 tential causes of these changes. For this purpose we roughly divided the history of the lake over the past Nothobranchiidae 2 century into two periods: (1) before and during the Poeciliidae 5 1980s and (2) after the 1980s. Latidae 1 Cichlidae/ haplochromines ca 500

LAKE VICTORIA AND ITS FISH DIVERSITY Cichlidae/ tilapiines 2 4 BEFORE AND DURING THE 1980S Anabantidae 1 Fig. 1. Outline figures of representatives of the fish families in Lake Victoria. a, Protopteridae; b, Mastacembelidae; c, Poeciliidae; Mastacembelidae 1 d, Latidae; e, Cichlidae; f, Nothobranchidae; g, Mochokidae; h, Anabantidae i, Schilbeidae; j, Clariidae; k, Bagridae; l, Alestidae; m, More than 500 endemic haplochromine cichlid Mormyridae; n, Cyprinidae (after van Oijen 1995). species, all maternal mouth brooders, are known from Lake Victoria (Greenwood 1974; Kaufman & nid Rastrineobola argentea (Pellegrin, 1904) made Ochumba 1993; Seehausen 1996; Witte et al. 2007a). an important contribution to the fish fauna in the lake Furthermore, Lake Victoria used to harbour two na- (Okedi 1974). tive tilapiine cichlids, and 46 native non-cichlid spe- Lake Victoria haplochromines have been classified cies (Table 1), of which 16 are endemic to the lake into 15 (sub)trophic groups, each consisting of spe- and its drainage basin (Greenwood 1974; van Oijen cies sharing morphological characters related to the 1995). The native non-cichlid species belong to 12 capture, uptake, and processing of their dominant families (Table 1; Fig. 1). In the 1950s, the Nile food source (Greenwood 1974; Witte & van Oijen perch, Lates niloticus (Linnaeus, 1758), Nile tilapia, 1990). The distribution of trophic groups is habitat Oreochromis niloticus (Linnaeus, 1758) and three dependent. For instance, epilithic algae grazers are other tilapiine species were introduced into the lake restricted to rocky shores; insectivores and oral shell- (Welcomme 1988; Pringle 2005). ing molluscivores are mainly associated with hard Figure 3. Haplochromine trophic groups from Lake Victoria: (a) spe- From the end of the 1960s till the beginning of the substrates like sand and rocks; and detritivores are concentrated near mud bottoms (e.g. Greenwood cies composition in all habitats. Note that the paedophages and the scale 1980s, the haplochromine cichlids made up more scraper are included in the piscivores and that rare trophic types, like the than 80% of the demersal fish catches (Fig. 2b; Kud- 1974; Witte 1981; Witte et al. 1992; Seehausen et al. parasite feeders and a crab eater, are not included; (b) biomass composi- hongania & Cordone 1974). Other prominent species 1997b). tion in sub-littoral waters (6-20 m deep; from Witte et al. 2009a). in bottom trawl catches were Oreochromis esculen- With respect to the total number of species, the tus (Graham, 1929), O. variabilis (Boulenger, 1906), piscivores and insectivores were the most common Clarias gariepinus (Burchell,1822), Protopterus ae- groups (Fig. 3a), however, the detritivores and zoo- thiopicus Heckel, 1851 and Bagrus docmak Forsskål, planktivores were the most important with respect to Fig. 2. Changes during the 1970s and 1980s in (a) total fish landings in the Tanzanian part of Lake Victoria and b( ) demersal fish stocks in the Mwanza Gulf (Tanzania) calculated from bottom trawls catches. The decline of haplochromines in the bottom trawl 1775 (Kudhongania & Cordone 1974; Goudswaard biomass, at least in the sub-littoral habitat (6-20 m deep; Fig. 3b), and probably also in the open waters catches in the Mwanza Gulf in the period 1973-1984 was mainly due to trawl fishery; the subsequent further decline was caused & Witte 1997; Goudswaard et al. 2002a, b). Apart by the Nile perch upsurge. Note that bottom trawls did not catch the small pelagic Rastrineobola argentea. After 1985, Nile tilapia from these demersal fishes, the small pelagic cypri- of the lake. dominated the tilapiine catches (after Witte et al. 1999). 52 Proceedings of the 4th International Conference on African Fish and Fisheries Lake Victoria’s Fish Fauna during the Past Century – Witte et al. 53

Several authors have raised doubts about the taxo- speciation through disruptive sexual selection for fishery in Lake Victoria. Perhaps with the exception 127 mm (5 inch) for gill nets was repealed in 1958, nomic status of Lake Victoria haplochromines (e.g. conspicuous coloration (Maan et al. 2004; Seehausen of scoop nets, netting material was not used in the which resulted in a short revival and subsequent fur- Sage & Selander 1975; Meyer 1987). However, et al. 2008) and strong assortative mating (Seehausen traditional fishing techniques in Lake Victoria (Gra- ther dwindling of the catches. The changes in the ecological research corroborated in many cases the & van Alphen 1998). Learning in the form of sexual ham 1929). Flax gill nets from Europe had been in- Lake Victoria fish stocks between 1950 and 1980 biological soundness of species distinction originally imprinting seems to facilitate assortative mating and troduced in 1905, but traps, baskets and moving pa- conform in a general way to the fishing down model, based on male colouration and small morphological reproductive isolation among closely related cichlid pyrus fences (operated as a kind of seine) were still viz. a dramatic decline in tilapiine catches, followed differences; viz. no indications of gene flow could be species (Verzijden & ten Cate 2007).Water clarity frequently used in 1928. Even in the 1970s and 1980s by a decline in large catfishes and lung fish and an in- found between presumed species that live in sym- appears to be important for this mode of speciation, traps were still in use in some areas. crease in smaller taxa including haplochromine cich- patry (Hoogerhoud et al. 1983; Goldschmidt & Witte and there is a significant correlation between the From the 1920s till the beginning of the 1950s, the lids (Balirwa et al. 2003; Welcomme 2005). 1990; Seehausen et al. 1998, 2008). number of coexisting haplochromine species and most important food fish of the lake was the tilapi- Fishery for the small pelagic R. argentea using lamps Molecular studies suggest that the 500+ haplochro- transparency among different East African lakes and ine cichlid O. esculentus, but other large fish species to attract the fish developed in the 1960s and 1970s in mine species of Lake Victoria evolved within the among localities within Lake Victoria (Seehausen et such as O. variabilis, the lungfish P. aethiopicus and Lake Victoria, and was derived from a similar fishery geologically short period of 100,000-400,000 years al. 1997a; Mrosso et al. 2004). the catfishes C. gariepinus and B. docmak were also in Lake Tanganyika aiming at clupeids (Okedi 1981). (Meyer et al. 1990; Nagl et al. 2000; Seehausen et Apart from sympatric speciation through disruptive fished (Graham 1929; Garrod 1960). In shallow wa- Originally, fish were attracted by lamps attached to al. 2003; Verheyen et al. 2003). However, paleolim- sexual selection, sympatric ecological speciation by ter, women caught the small (< 10 cm total length, rafts that were hauled in slowly to the shore, where nological data indicate that Lake Victoria was com- disruptive natural selection for resources may have TL) cyprinid R. argentea by forming a circle, driving the fish were caught with small meshed beach seines. pletely dry between 18,000 and 15,000 years ago played a role. A potential example concerns the spe- the fish to the centre and scooping them from the wa- By the end of the 1980s, lift nets and encircling nets, (Johnson et al. 1996; Stager & Johnson 2008). The cies pair H. piceatus Greenwood & Gee, 1969 and ter with baskets (Graham 1929). which could be operated offshore, were used to catch discrepancy between these data resulted in vigorous H. coprologus Niemantsverdriet & Witte, 2010. Apart from the widely appreciated O. esculentus, the the fish that had been attracted by the lamps (Ligtvoet debates about the origin, age, and evolutionary his- These species have a similar male colouration, but preference for other fish species in Lake Victoria dif- et al. 1995). tory of the extraordinary speciose Lake Victoria hap- H. coprologus, which used to feed on detritus and fered locally. For instance, because of its snake-like A lake wide trawl survey in 1969 estimated the stand- lochromines (e.g. Nagl et al. 2000; Seehausen et al. phytoplankton, was more deep bodied and had a appearance, the lungfish was disliked by most- Wa ing stock of haplochromine cichlids at 600,000 t (80 % 2003; Verheyen et al. 2003; Fryer 2004; Seehausen longer intestine than H. piceatus with a diet of zoo- sukuma people living on the south-eastern shores of of the demersal fish stock in the lake), and it was sug- 2006; Elmer et al. 2009). Nagl et al. (2000) suggest- plankton and insect larvae (De Zeeuw et al. 2010). the lake, but it was highly appreciated by the Waju- gested that 200,000 t y-1 could be harvested (Kud- ed that the Lake Victoria haplochromines originated Furthermore, they differed in depth distribution and luo living around the Nyanza Gulf. The Wahaya and hongania & Cordone 1974). Ways were sought to from trophic generalists, which lived in the East- spawning sites (Goldschmidt et al. 1990, 1993; De Baganda on the western and northern side of the lake exploit this major fish source. Bottom trawling for African river systems and in which mutations for Zeeuw et al. 2010). Sympatric ecological speciation considered B. docmak a delicacy. Haplochromine haplochromines as supply for a fishmeal factory in morphological adaptations were already present as by disruptive natural selection for resources has also cichlids were popular as food fish among the -Wak Mwanza started in the Tanzanian waters in 1976. polymorphisms. Data of Seehausen et al. (2003) indi- been suggested for the cichlid species flock in the erewe at Ukerewe Island (Graham 1929; Witte et al. This factory converted some 10-15 t of haplochro- cated that the Lake Victoria –Edward flock is derived Crater Lake Barombi Mbo in Cameroon (Schliewen 1999). mines per day into animal fodder, and signs of local from the morphologically and ecologically diverse et al. 1994) and for the Lake Tana barbs (Sibbing et over-fishing of haplochromines in the Mwanza area cichlid genus Thoracochromis from the Congo and al. 1998). Finally, allopatric speciation in satellite Fishing down the food web were reported within a few years (Fig. 2b; Witte & Nile. Verheyen et al. (2003) explained the fast radia- lakes, that later became connected again to the main The introduction of modern fishing gear, like gill Goudswaard 1985). tion of eco-morphological diversity in Lake Victoria lake, probably contributed to species diversity as nets and beach seines, and the increased demands for haplochromines by their descent from the lacustrine, well (Greenwood 1965, 1974; Kaufman et al. 1997). fish because of the growing human population and Species introductions possibly already diversified, Lake Kivu ancestors, a Seehausen (2000) stressed that ‘the evolution of spe- the opening of new markets, due to new roads and To improve the dwindling catches, several fish spe- view confirmed by Elmer et al. (2009). All these pa- cies diversity requires three processes: speciation, railway connections, had a strong impact on the fish cies were introduced into Lake Victoria in the 1950s pers suggest that the Lake Victoria cichlid flock sensu ecological radiation and anatomical diversification…’. catches in the first half of the 20th century (e.g. Gra- (Welcomme 1988). They comprised the Nile perch stricto must be older than 15,000 years, and is not The functional decoupling of the upper and lower pha- ham 1929; Beverton 1959; Fryer & Iles 1972; Balir- (L. niloticus), Nile tilapia (O. niloticus) and the tila- strictly monophyletic. Elmer et al. (2009) estimated ryngeal jaws in cichlid fish may have contributed to wa et al. 2003; Balirwa 2007). Popular food fish such piines O. leucostictus (Trewavas, 1933), Tilapia zillii that the most recent common ancestor of the cich- the third process and may explain the high degree tro- as O. esculentus and the cyprinid Labeo victorianus (Gervais, 1848) and T. rendalli Boulenger, 1896. In lids of lakes Victoria, Albert, Edward, George, Kivu phic radiation of the haplochromines (Galis & Drucker Boulenger, 1901, showed clear signs of over-fishing the 1980s Nile perch suddenly boomed in Lake Vic- and Kyoga together (the Lake Victoria region ‘super 1996). However, as Seehausen (2000) suggested, the by the 1940s and 1950s respectively (Cadwalladr toria and, concomitantly, the haplochromine cichlids flock’) existed about 4.5 million years ago. They high degree of phenotypic plasticity in cichlids (e.g. 1965; Fryer & Iles 1972; Fryer 1973). The introduc- in the sub-littoral and offshore areas vanished almost also suggest that the Pleistocene desiccation ‘bottle- Witte et al. 1997), possibly also played a role. tion, after the Second World War, of the more catch completely (Fig. 2; Barel et al. 1985, 1991; Ogutu- necked but did not extirpate’ the adaptive radiation of efficient and long lasting nylon gill nets, and of out- Ohwayo 1990a; Witte et al. 1992). These included Lake Victoria haplochromines. Human induced changes board engines, further increased fishing pressure. The areas where haplochromines had been fished and Rapid speciation has been suggested to be a typical Fishery in Lake Victoria before the Nile perch boom catch per night of native tilapiines in a 50 m-long 127 already declined, but also areas where there was no feature of haplochromine cichlids (Seehausen 2006) In a report on a lake-wide expedition in 1928, Gra- mm-mesh gill net, decreased from 50-100 fish in fishery on haplochromines. Other species, like the in- and is thought to be mainly the result of sympatric ham (1929) provided an extensive description of the 1905 to < 0.5 fish in the same net in 1970 (Kudhon- troduced Nile tilapia and especially the native R. ar- gania & Cordone 1974). The minimum mesh size of gentea increased in biomass in the presence of the 54 Proceedings of the 4th International Conference on African Fish and Fisheries Lake Victoria’s Fish Fauna during the Past Century – Witte et al. 55

Nile perch (Wanink 1999; Goudswaard et al. 2002b). eradicating haplochromines that were potential preda- plankton grazing by fish (Witteet al. 2012). species, including those living along rocky shores The biomass of R. argentea increased approximately tors and competitors of juvenile Nile perch, but see Eutrophication and algal blooms caused decreases (Seehausen et al. 1997a, 2008; Seehausen 2006). by a factor of four, but the increase in numbers was Downing (2012). However, in some areas of the lake in water transparency (Mugidde 1993, Seehausen et Lake Victoria haplochromines are rather tolerant of about double as high due to a decrease in size of this this seems to have been done by heavy fishing on the al. 1997a; Witte et al. 2005) and in dissolved oxy- low oxygen concentrations (Verheyen et al. 1986; small cyprinid (Wanink 1999). This strong increase haplochromines (Goudswaard et al. 2008). gen levels (Ochumba & Kibaara 1989; Kaufman Chapman et al. 1995; Rutjes et al. 2007), therefore in catch may have been caused by competitive re- Thirty years after its introduction, Nile tilapia had be- 1992; Hecky et al. 1994; Wanink et al. 2001). Sud- the impact of the increased hypoxic conditions due lease with the former abundant (zooplanktivorous) come the most common tilapiine species in Lake Vic- den upwelling of hypoxic water caused mass fish to eutrophication (Hecky et al. 1994) may have been haplochromines and a reduction in generation time toria. It replaced the overfished native tilapiines almost kills (Ochumba & Kibaara 1989; Ochumba 1990; less severe than suggested (Witte et al. 2005). (Wanink & Witte 2000a). Moreover, in contrast to completely before the Nile perch came to dominate Kaufman 1992; Kudhongania & Chitamwebwa Some satellite lakes of lakes Victoria and Kyoga haplochromines, R. argentea were mainly eaten by the ecosystem (Ogutu-Ohwayo 1990a; Goudswaard 1995; Wanink et al. 2001; Goudswaard et al. 2011). were not invaded by Nile perch or affected by eutro- Nile perch in shallow water (< 12 m deep) but not et al. 2002b). The main cause of the final disappear- The wetland zone that regulates material transport to phication. Apart from endemic haplochromine spe- in the deeper parts of the lake (Katunzi et al. 2006). ance of the native tilapiine species is presumed to be the lake has also been under intense pressure from cies, these satellite lakes contain some species that A strong increase was also observed in the biomass the competitive dominance of Nile tilapia (Lowe- human activities (Balirwa 1995; Kairu 2001). Clear- are the same as, or similar to, those that vanished of the shrimp Caridina nilotica, possibly due to a McConnell 2000, Goudswaard et al. 2002b). ing of papyrus (Cyperus papyrus Linnaeus 1753) from the main lakes; thus providing important refu- decreased predation pressure on juvenile shrimps In 1989, the South American water hyacinth, Eich- fringes along the lake shore and conversion of wet- gia for some species and trophic groups (Kaufman by haplochromines (Goldschmidt et al. 1993; Goud- hornia crassipes (Martias) Solms, 1883 appeared in lands into agriculture land may have contributed to & Ochumba 1993; Mwanja et al. 2001; Aloo 2003; swaard et al. 2006; Budeba & Cowx 2007b). Lake Victoria. This new intruder quickly established the decline of lungfish by destroying their breeding Mbabazi et al. 2004). It took more than 25 years since the first release of itself (Njuguna 1991). For almost a decade exten- habitat (Goudswaard et al. 2002a). It has been sug- Nile perch predation, competition with introduced Nile perch in Lake Victoria before a dramatic upsurge sive mats of this weed covered large areas along the gested that water hyacinth mats, which are common species, habitat deterioration and fishing pressure also in the abundance and catch landings of Nile perch lake’s shores, depriving waters of light and oxygen. since the 1990s, provided a suitable habitat for lung- contributed to declines in other native species includ- was observed. This may have been driven, at least in To eradicate the weed, locally mechanical removal fish and that it played a role in their resurgence in the ing catfishes [B. docmak, Xenoclarias eupogon (Nor- part, by high survival of very young Nile perch and was applied. Furthermore, South American weevils second half of the 1990s (Bugenyi & Van der Knaap man, 1928), Synodontis victoriae Boulenger, 1906], the food abundance for these fishes. Goudswaard et (Neochetina eichhorniae Warner, 1970 and N. bru- 1997). the lungfish Protopterus aethiopicus, the mormyrid al. (2008), suggested that the abundant haplochro- chi Hustache, 1926), were introduced for biological Climate change may also have been one of the stress- Mormyrus kannume Forsskål, 1775, the cyprinid mines fed upon eggs and larvae of Nile perch, when- control of the water hyacinth. In 1998 the water hya- ors that affected Lake Victoria (Hecky et al. 2010). Barbus altianalis radcliffi Boulenger, 1903 and the ever available. Moreover, the haplochromines were cinth strongly declined again. However, according In the 1990s, the lake was warmer than in the 1960s tilapiines O. esculentus and O. variabilis (Table 2; probably competing with juvenile Nile perch for zoo- to some authors this decline was not only due to the with a shallower, more stable and more persistent Goudswaard 1988; Ogutu-Ohwayo 1990a; Goud- plankton and insect larvae. In contrast, a recent study weevils, but also to El Niño events during 1997/1998 thermocline, which contributed to the deoxygen- swaard & Witte 1997; Goudswaard et al. 2002a, b). found that the timing and speed of the Nile perch up- that resulted in extensive clouds, which reduced the ation of deep water (Hecky 1993; Hecky et al. 1994, However, the impacts of the above described human surge was not controlled by external triggers and sim- light conditions and consequently plant growth (Wil- 2010). Low wind stress that was measured from the induced threats were not the same for all fish species. ply grew exponentially (Downing 2012). Nile perch liams et al. 2005, 2007). Moreover, shore bound mats mid 1970s to the mid 1990s in Lake Victoria could In several cases a threat to one species did have a first became noticeably successful in the heavily ex- were dislodged by raising water levels and driven to also have contributed to the longer and more stable positive effect on another species; e.g. the decline of ploited Nyanza Gulf, Kenya, in 1980. By the early open water, where waves helped to destroy them. In anoxic layers (Kolding et al. 2008). haplochromines by Nile perch predation had a posi- 1970s, over-exploitation of haplochromines was al- the Mwanza Gulf densities of water hyacinth have tive effect on the population of R. argentea (Table 2). ready evident in this area (Marten 1979), which may fluctuated during the past decade, but they have not LAKE VICTORIA AFTER THE 1980S have facilitated increased survival of juvenile Nile returned to the peak levels observed in the 1990s Resurgence of some haplochromine species perch. During the subsequent expansion from the (MAKM, JHW and FW pers. obs.). Decline of fish diversity In the course of the 1990s, after a decline of Nile Nyanza Gulf towards other areas of the lake, the adult The boom of the introduced Nile perch in the 1980s as perch in Lake Victoria due to intensive fishing, a and sub-adult Nile perch fed heavily upon haplochro- Environmental degradation and climate change well as fishery and habitat deterioration had a strong slow resurgence of some haplochromine species was mine cichlids (Hughes 1986; Ogutu-Ohwayo 1990a, In the course of the 1980s, frequent blooms of cya- impact on the haplochromine cichlids and many observed in the sublittoral waters (Witte et al. 2000, b; Mkumbo & Ligtvoet 1992) and reduced their num- nobacteria also became a common feature. They were other fish species (Ogutu-Ohwayo 1990a; Witteet al. 2007a, b; Balirwa et al. 2003; Getabu et al. 2003). bers to extremely low levels. This process, in some the result of eutrophication due to the increased hu- 1992; Goudswaard & Witte 1997; Goudswaard et al. Similar observations were made in Lake Nabugabo, areas like the Mwanza Gulf accompanied by over- man population density, deforestation, and agriculture 2002a, b, 2008). It was estimated that some 200 of the a shallow satellite lake of Lake Victoria (Ogutu- fishing of haplochromines, likely enhanced survival (Hecky 1993; Mugidde 1993; Scheren et al. 2000; Ver- endemic haplochromine species may have gone ex- Ohwayo 1993; Chapman et al. 2003). of eggs and larvae of Nile perch. After the decline of schuren et al. 2002). Losses of phytoplankton through tinct (Witte et al. 1992). The highly structured rocky The resurgence in Lake Victoria, mainly concerned the haplochromines and the upsurge of the shrimps, grazing by fish were less than 5% of daily gross and shores and papyrus fringes, where Nile perch densi- zooplanktivorous and detritivorous haplochromines, juvenile shrimps became an important food item for less than 15% of daily net phytoplankton production. ties are low, were less affected by Nile perch than but of each group only about 30% of the species recov- small (< 10 cm TL) Nile perch (Katunzi et al. 2006; As a consequence it is unlikely that the phytoplankton the sub-littoral and offshore waters (Witte et al. 1992, ered, and initially the ratio in biomass of detritivores Goudswaard et al. 2006). Thus, adult Nile perch may blooms in the second half of the 1980s were due to a 2007a; Seehausen 1996). However, the decrease in and zooplanktivores was reversed (Witte et al. 2007a, have facilitated the survival of their offspring by top-down effect caused by the strong decline in phyto- water transparency due to eutrophication may have b). Before the 1980s detritivores made up about 50% caused hybridization among several haplochromine of the haplochromine biomass in the sublittoral waters 56 Proceedings of the 4th International Conference on African Fish and Fisheries Lake Victoria’s Fish Fauna during the Past Century – Witte et al. 57

Table 2. Human induced changes and their impacts on various fish stocks (neg = negative; pos = positive). of the Mwanza Gulf and zooplanktivores about 25% (Paralabidochromis) plagiodon Regan & Trewavas, (Fig. 3b; Goldschmidt et al. 1993), whereas by 2005 1928, which formerly were restricted to the shal- Change impact on: detritivores constituted only 26% and zooplanktivores low (< 6 m) sand stations in Butimba Bay. Since the Increased fishery neg Most fish species (esp. native tilapiines + large cyprinids) and finally on Nile more than 70% (Witte et al. 2007a). In 2008 and in resurgence of the species in the 1990s, both oc- perch February - April 2011, detritivores were the dominant cur predominantly over mud bottoms up to 11 m Increased Nile perch neg Most fish species (esp. haplochromines), but not on R. argentea and Nile group again (Kishe-Machumu 2012; MAKM, JCVR depth (Seehausen et al. 1997b; Kishe-Machumu tilapia and FW pers. obs.). However, the majority of the spe- 2012; MAKM, JHW, JCVR and FW unpublished Increased Nile tilapia neg Native tilapiines cies did not recover (Witte et al. 2007a). In spite of data). In contrast the zooplanktivore Haplochromis frequent sampling in the Mwanza Gulf in the period (Yssichromis) pyrrhocephalus Witte & Witte-Maas, Habitat deterioration neg Lungfish + C. gariepinus? between 1987 and 2011, many of the highly special- 1987 now also occurs in shallower areas than in the past. Increased eutrophication neg Especially haplochromines (hybridization) ized trophic types like scale eaters, parasite eaters and Reproductive strategies changed in the Nile tilapia Increased water hyacinth pos Lungfish and C. gariepinus and possibly some other fish species that could prawn eaters have not been caught, whereas piscivores (Ojuok et al. 2007) and the cyprinid R. argentea cope with low oxygen levels and limited light conditions and paedophages are extremely rare now, both with (Wanink & Witte 2000a; Manyala & Ojuok 2007); Decreased haplochromines pos Juvenile Nile perch, shrimps and R. argentea respect to numbers of individuals and species. Similar both have shown a decrease in their size at matu- results were found by Mizoiri et al. (2008) who sam- rity. In the zooplanktivorous haplochromines, Hap- pled the Mwanza Gulf and the Speke Gulf at many lochromis (Yssichromis) laparogramma Greenwood localities in 2004, 2005 and 2006. & Gee, 1969, H. pyrrhocephalus and H. tanaos, Nile perch longer than 20 cm total length, which an increase of both absolute and relative fecundity since the disappearance of the haplochromine cichlids have been observed (Wanink 1991; Wanink & Witte mainly fed on shrimps, its own juveniles and R. argen- 2000a; JHW unpublished data). tea (Hughes 1986; Ogutu-Ohwayo 1990b; Mkumbo Dietary shifts were observed in zooplanktivorous and & Ligtvoet 1992; Katunzi et al. 2006), switched again detritivorous/phytoplanktivorous haplochromines Table 3. Preliminary data on ecological and morphological changes in resurging detritivorous (Detr), zooplanktivorous to haplochromines after their re-emergence (Budeba (van Oijen & Witte 1996; Katunzi et al. 2003; Kishe- (Zoo) and oral shelling molluscivorous (Or sh) haplochromine species. Numbers represent the number of species in which the changes were observed. Note that absence of changes may imply that no changes were observed or that the trait has & Cowx 2007a; Kishe-Machumu et al. 2011). Machumu et al. 2008; van Rijssel et al. submitted) not yet been studied. as well as in the oral shelling haplochromine Platy- Responses to environmental changes taeniodus degeni Boulenger, 1906 (van Rijssel et al. Apparent responses to the environmental changes submitted). In all these cases the amount of macro- Trophic group Detr Zoo Or sh Sources were observed in several fish species in the Mwanza invertebrates in the diets increased strongly. Similar (number of species) (2) (3) (1) Gulf. Some haplochromine species extended their changes in diet were found in other fish taxa (Table 4). Trait use of habitat (Table 3). Most striking were the zoo- These changes were also reflected in comparisons of Habitat change 2 3 1 1, 2, 3 planktivorous Haplochromis (?) tanaos van Oijen stable isotopes of detritivorous and zooplanktivorous & Witte, 1996 and the snail shelling Haplochromis haplochromines between the pre- and post-Nile perch Increased fecundity - 3 - 4, 5 Diet 2 3 1 1, 6, 7, 8, 9 Table 4. Dominant food items in diets of different fish taxa from Lake Victoria before and after the ecological changes in the 1980s. Intestine length 2 - - 8 Head volume - 3 - 10, 11 Taxon before 1980s 1990s -2006 Body shape 1 3 1 12 Zooplanktivorous hapl. (3 sp.)1,2,3 zooplankton Macroinvertebrates (+zoopl) Detritivorous hapl. (> 3 sp.)4 detritus/phytoplankton macroinvertebrates Eye size - 3 1 10, 12, 13 Platytaeniodus degeni3 detritus (+ molluscs) macroinvertebr (+ molluscs) Retina - 2 - 13, 14 Bagrus docmak5 haplochromines insects, R. argentea Muscles feeding app - 1 - 10 Schilbe intermedius (Linnaeus, 1758)5 haplochromines insects Dentition premaxilla - 2 1 9 Rastrineobola argentea6,7 zooplankton macroinvertebr, fish Gill raker number - 1 - 11 Brycinus sadleri (Boulenger, 1906)8 plants, insects macroinvertebrates Gill surface area - 2 1 10, 11 Nile tilapia7,9,10,11 detritus/phytoplankton macroinvertebrates

1 2 3 4 5 Sources = 1van Oijen & Witte 1996; 2Seehausen et al. 1997b; 3Kishe-Machumu 2012; 4Wanink 1991; 5JHW unpublished; van Oijen & Witte 1996; Katunzi et al. 2003; Van Rijssel et al. submitted; Kishe-Machumu et al. 2008; Olowo & 6 7 8 9 10 6Wanink 1998; 7Katunzi et al. 2003; 8Kishe-Machumu et al. 2008; 9Van Rijssel et al. submitted; 10Witte et al. 2008; 11 JCVR Chapman 1999; Wanink 1998, Budeba & Cowx 2007a; Wanink & Joordens 2007; Gophen et al. 1993; Bwanika et al. 11 unpublished; 12Van Rijssel & Witte 2013; 13Van der Meer et al. 2012; 14Witte et al. 2005. 2006; Njiru et al. 2007. 58 Proceedings of the 4th International Conference on African Fish and Fisheries Lake Victoria’s Fish Fauna during the Past Century – Witte et al. 59

era (Kishe-Machumu 2012). In the detritivorous/phy- blue light is filtered out faster than red and green light. total fish landings reflected an increase in fishing effort, toplanktivorous haplochromines the shift in diet was So, blue sensitive cones may be of reduced relevance which was indeed observed during the past decades accompanied by a decrease of 30% in relative intestine in the current visual habitat of H. tanaos. In spite of (Ogutu-Ohwayo 2004; Matsuishi et al. 2006; Balirwa length (Kishe-Machumu et al. 2008). As discussed by its reduced eye size, light sensitivity was probably 2007). The number of fishers and fishing boats tripled Kishe-Machumu et al. (2008) potential factors for improved by the increased size of the double cones, between 1990 and 2007 (Fig. 4; Ogutu-Ohwayo 2004; these diet shifts could be: (a) the increased availability but at the cost of the resolving power. However, as it Matsuishi et al. 2006; Mkumbo et al. 2007). of profitable food items; (b) the loss of competitors, currently feeds on larger prey, the loss of resolution Since the1980s, the total annual landings in the lake and (c) the increased water turbidity after the environ- may not have much impact on feeding performance increased, but by the mid 1990s the contribution of mental changes. In the last case, it is supposed that the of H. tanaos. Finally, preliminary studies suggest Nile perch showed some decline, whereas landings of fish that grew up under low light conditions changed changes in oral dentition in several of the recovering R. argentea and Nile tilapia increased (Fig. 4; Matsui- Fig. 4. Trends in total annual fish landings and total fishing effort their retina in such a way that they increased their light species, which also may be related to the change in shi et al. 2006). Just after the boom, Nile perch con- in Lake Victoria during the period 1975 – 2005. Nile tilapia is sensitivity at the cost of their resolution (e.g. Van der diet (Van Rijssel et al. submitted) tributed more than 70% to the fish landings (Figs 2, included in ‘other species’. Note that the increase in effort was mainly directed at Nile perch (effort data from LVFO Frame Sur- Meer 1993; Van der Meer et al. 2012). Though less striking than in the haplochromine cich- 4; Van der Knaap et al. 2002), but between 1990 and vey Report, 2006; Catch data from LVFO IFMP 2, 2008; Figure Some resurgent zooplanktivorous species showed lids, morphological changes were also observed 2000 the catch per unit effort of Nile perch dropped from Witte et al. 2009b). body shape changes as a response to the environmen- in the cyprinid R. argentea; the number of gill fila- from about 80 to 45 kg per boat per day in the Kenyan tal changes while other zooplanktivores, which are ments increased, whereas the number of gill rakers waters, where adequate data had been collected (Mat- (Ligtvoet 1989; Ligtvoet et al. 1995). The smaller thought to be extinct or poorly recovered, showed decreased, possibly in response to the lower oxygen suishi et al. 2006). By 2000 the total annual landings ones were dried in the sun or smoked. For frying and changes in the opposite direction (Van Rijssel & Witte concentrations and larger prey, respectively (Wanink amounted to 657,000 t, 40% of which was made up smoking firewood was needed, thus the Nile perch 2013). One of the resurgent species is the zooplank- & Witte 2000b). by Nile perch, 41% by R. argentea and 8% by Nile boom strongly exacerbated the ongoing deforestation tivorous H. pyrrhocephalus which is currently the The rapid morphological changes described above tilapia (calculated from Table 1 in Matsuishi et al. along the lake shore. In the 1990s filleting factories most common haplochromine cichlid in the Mwanza may reflect environmentally induced plasticity, herita- 2006). The landings of haplochromine cichlids had arose for export of Nile perch fillets to Europe and Gulf, and its morphology has been studied extensive- ble response to natural selection, genetic introgression increased from virtually zero at the end of the 1980s to Asia (Ntiba et al. 2001). The total capacity of these ly. A comparison of specimens collected in the 1970s through hybridisation or, most likely, a combination 17,000 t y-1 in the Tanzanian waters and to 4,000 t y-1 factories is several hundred tons per day, and they (pre-Nile perch population) and those collected in the of several of these factors. Recent studies of contem- in the Ugandan waters, whereas in Kenyan waters the became the main buyers of Nile perch. Many of these 1990s (modern population) revealed that head length porary evolution in natural populations, especially in catches were only 300 t y-1 (Matsuishi et al. 2006). fish processing plants now operate below their in- and head volume (for H. pyrrhocephalus) decreased response to environmental changes caused by human In the period 2005-2007, the annual landings were stalled capacity. Balirwa (2003) reports that in Ugan- in three different haplochromines (Witte et al. 2008; activity, yielded estimates of potential rates of evolu- even estimated at 1 million t and the contribution of da, 15 factories with a total installed capacity of 420 t Van Rijssel & Witte 2013). These size decreases tion many orders of magnitude greater than rates in- Nile perch was about 26%, while that of R. argen- per day, are actually processing 185 t per day. Current- match biomechanical predictions for increased swim- ferred from the fossil record (e.g. Carrol et al. 2007). tea had increased to about 53% (Fig. 4, LVFO, CAS ly, about 1.2 million people are directly or indirectly ming speed in presence of predators (Langerhans et al. Report 2006). Apparently, the species composition in dependent for their livelihood on the fishery in Lake 2004, Chapman et al. 2008). The gill surface area in Fishery in Lake Victoria after the Nile perch boom the fish landings has changed toward lower trophic Victoria (Matsuishi et al. 2006). In 2003 the estimated resurgent H. pyrrhocephalus increased by 64%, which By the end of the 1980s only three fish species were level species (viz. R. argentea and Nile tilapia, Mat- annual catch was worth at least US$ 540 million at the is an apparent adaptation to the increased hypoxic common in sub-littoral and offshore waters of Lake suishi et al. 2006). Hydroacoustic surveys between fish landings, whereas a further US$ 240 million was conditions. The gill rakers decreased in length and Victoria. These were the small indigenous cyprinid 1999 and 2008 suggested that over the studied period the cheek depth and the musculus levator posterior, R. argentea and the introduced Nile perch and Nile the overall fish biomass in Lake Victoria remained responsible for biting force of the pharyngeal jaws, in- tilapia (Fig. 2a; Ogutu-Ohwayo 1990a; Wanink 1999; more or less constant, but the biomass of Nile perch creased in size (Witte et al. 2008). These changes may Goudswaard et al. 2002b). Together, they dominated decreased, whereas that of R. argentea increased reflect adaptive responses to the larger and tougher the fish landings by more than 80% (Fig. 2a; Reyn- (Fig. 5; Getabu et al. 2003; Mkumbo et.al 2005; prey types in the diet of modern H. pyrrhocephalus olds et al. 1995). LVFO Hydroacoustic Survey Report 2007; LVFO, (Katunzi et al. 2003). Reductions in eye size and in Although biodiversity decreased strongly and water IFMP 2, 2008; Kayanda et al. 2011). The foregoing the size of the musculus sternohyoideus, and realloca- quality deteriorated, fish production in Lake Victoria seems to represent a second fishing down episode in tion of space among compartments of the head seem flourished after the Nile perch boom. In the 1960s, Lake Victoria (Balirwa et al. 2003). to have permitted accommodation of larger gills in a the total landings for the lake were approximately Originally, the fishermen did not like Nile perch be- relatively smaller head (Witte et al. 2008). 100,000 t y-1. In the late 1980s and early 1990s, just cause they had problems with handling, processing A reduction in eye size was also observed in H. tanaos. after the Nile perch boom, the fisheries produced over and marketing the fish; the larger and relatively fat However, it was also observed that while the density 500,000 t of fish annually (Balirwa 2007). Concomi- perch could not easily be dried or transported. How- Fig. 5. Estimated fish biomass in Lake Victoria based on acous- of the blue sensitive single cones were reduced, the tantly, bottom trawl catches revealed that the standing ever, in the years after the upsurge, people rapidly tic surveys in the periods 1999-2001 and 2005-2007. Note size of the red and green cones in the retina had in- stock of demersal fish had decreased about five times adjusted the processing and transport techniques. The that the equipment and analytical protocols differed between creased (Witte et al. 2005; Van der Meer et al. 2012). (Fig.2 b; Okaronon 1994; Witte et al. 1999; Balirwa larger fishes were chopped into pieces and subse- the periods1999–2001 and the period 2005-2007 (after LVFO, In eutrophied water with high algae concentrations, 2007). This indicates that the dramatic increase of the quently fried in the fat removed from the intestines Hydroacoustic survey Reports 2006 & 2007; Figure from Witte et al. 2009b). 60 Proceedings of the 4th International Conference on African Fish and Fisheries Lake Victoria’s Fish Fauna during the Past Century – Witte et al. 61

earned in fish exports (Balirwa 2007). ment of Lake Victoria. A modelling study suggested and the species culture potential: lessons to learn’. East Africa’. Conservation Biology 17: 500-511. Several observations, e.g. the decline in annual land- that Nile perch prefer and grow fastest on a haplo- African Journal of Ecology 45: 120-129. Chapman, L. J., Kaufman, L.S., Chapman C.A. & McK- ings and the decline in size at first maturity, suggest chromine prey base (Kaufman & Schwarz 2002). If Balirwa, J.S., Chapman, C.A., Chapman, L.J., Cowx, I.G., enzie, F.E. 1995. ‘Hypoxia tolerance in twelve spe- that Nile perch is intensively fished, which may result the model is realistic, it would suggest that it is worth Geheb, K., Kaufman, L., Lowe-McConnell, R.H., cies of East African cichlids: potential for low oxy- in overexploitation. Consequently, it has been sug- thinking of management strategies that allow suffi- Seehausen, O., Wanink, J.H., Welcomme, R.L. & gen refugia in Lake Victoria’. Conservation Biolo- gested that, under the scenario of increased fishing cient fishing on Nile perch to ensure an abundance of Witte, F. 2003. ‘Biodiversity and fishery sustainabil- gy 9: 1274-1287. ity in the Lake Victoria basin: an unexpected mar- De Zeeuw, M.P., Mietes M., Niemantsverdriet, P., Ter effort, the Nile perch fishery is unsustainable (Pitcher their haplochromine prey, but not so much pressure riage?’ BioScience 53: 703-715. Huurne, S. & Witte, F. 2010. ‘Seven new species & Bundy 1995; Mkumbo 2002; Matsuishi et al. 2006; as to threaten the Nile perch stock itself (Balirwa et Barel, C.D.N., Dorit. R., Greenwood, P.H., Fryer, G., of detritivorous and phytoplanktivorous haplochro- Mkumbo et al. 2007; Kayanda et al. 2011). However, al. 2003). However, to allow maintenance and resto- Hughes, N., Jackson, P.B.N., Kawanabe, H., Lowe- mines from Lake Victoria’. Zoölogische Mededelin- according to Kolding et al. (2008) not over-fishing, ration of haplochromine diversity and of other fish McConnell, R.H., Nagoshi, M., Ribbink, A.J., Trewa- gen Leiden 84: 201-250. but the ongoing eutrophication is the main threat to species, the urgent measures must include serious at- vas, E., Witte, F. & Yamaoka, K. 1985. ‘Destruction Downing, A.S. 2012. ‘Was Lates late? A null model for the Nile perch fishery. Based on Chl α measurements tempts to reverse the eutrophication of Lake Victoria of fisheries in Africa’s lakes’. Nature 315: 19-20. the Nile perch boom in Lake Victoria’. In Seeing the by Silsbe et al. (2006) and annual Nile perch catches, (Seehausen et al. 1997a; Balirwa et al. 2003; Witte et Barel, C.D.N., Ligtvoet W., Goldschmidt T., Witte F. & water for the fish: building on perspectives of Lake Kolding et al. (2008) suggest that the top of the pro- al. 2005; Kolding et al. 2008). Goudswaard P.C. 1991. ‘The haplochromine cich- Victoria. PhD thesis, Wageningen University. ductivity curve for Nile perch as a function for eutro- lids of Lake Victoria: an assessment of biological Downing, A.S., Van Nes, E.H., Janse, J.H., Witte, F., Cor- phication has been reached. ACKNOWLEDGEMENTS and fisheries interest’. In M.H.A. Keenleyside (ed.), nelissen, I.J., Scheffer, M. & Mooij, W.M. 2012. In the past, the management measures governing Cichlid Fishes: Behaviour, Ecology and Evolution. ‘Collapse and reorganization of a food web of We thank our colleagues from the Haplochromis Lake Victoria resources were different in each coun- London: Chapman & Hall, pp. 258-279. Mwanza Gulf, Lake Victoria’. Ecological Applica- Ecology Survey Team (HEST), the Tanzania Beverton, R.J.H. 1959. Report on the State of the Lake Vic- tions 22: 229-239 try (Ntiba et al. 2001). Through the Lake Victoria Fisheries Research Institute (TAFIRI), the Lake Vic- toria Fisheries. Lowesoft: Fisheries Laboratory. Elmer, K.R., Reggio, C., Wirth, T., Verheyen, E., Salz- Fisheries Organization (LVFO) that was formed in toria Fisheries Organization (LVFO), the National Budeba, Y.L. & Cowx, I.G. 2007a. ‘The role of the fresh- burger, W. & Meyer, A. 2009. ‘Pleistocene desicca- 1994 and support from the EU to implement a Man- Fisheries Resources Research Institute (NaFIRRI, water shrimp Caridina nilotica (Roux) in the diet of tion in East Africa bottlenecked but did not extirpate agement Plan, attempts to harmonize policies and Uganda), for their co-operation. We are indebted to the major commercial fish species in Lake Victoria, the adaptive radiation of Lake Victoria haplochro- regulations and also to develop standard operational three anonymous reviewers for their constructive Tanzania’. Aquatic Ecosystem Health & Manage- mine cichlid fishes’. Proceedings of the National procedures are continuing. Lake-wide management comments. We wish to thank Martin Brittijn and Bas ment 10: 368-380. Academy of Science of the United States of Ameri- regulations are established, and a co-management Budeba, Y.L. & Cowx, I.G. 2007b. ‘Contribution of ca 106: 13404-13409. Blankenvoort for making the figures. The research by approach has been adopted. About 1060 Beach Man- Caridina nilotica (Roux) in the dagaa fishery in Lake Fryer, G. 1973. ‘The Lake Victoria Fisheries: some facts HEST was financially supported by The Netherlands agement Units (BMUs) have been established around Victoria, Tanzania’. Aquatic Ecosystem Health & and fallacies’. Biological Conservation 5: 304-308. Foundation for the Advancement of Tropical Research Lake Victoria to take an active role in management Management 10: 381-391. Fryer, G. 2004. ‘Speciation rates in lakes and the enigma (WOTRO), the Section for Research and Technology of the resources at beach level (LVFO Website www. Bugenyi, F.W.B. & van der Knaap, M. 1997. ‘Lake Vic- of Lake Victoria’. Hydrobiologia 519: 167-183. of The Netherlands Minister of Development Co- lvfo.org). The harmonized rules include: banning of toria: an example of ACP-EU fisheries research’. Fryer, G. & Iles, T.D. 1972. The Cichlid Fishes of the operation, the Schure-Beijerinck-Popping Fonds, the beach seines, bottom trawls and cast nets, and of gill European Commission Fisheries Cooperation Bul- Great Lakes of Africa. Edinburgh: Oliver & Boyd. International Foundation for Sciences (IFS), the van letin 10: 20-21. Galis, F. & Drucker, E.G.1996. ‘Pharyngeal biting me- nets below 13 cm (5 inches) mesh size; as well as Tienhoven Stichting and Yellow Springs Instruments. Bwanika, G. N. Chapman L.J. & Kizito, Y. 2006. ‘Cascad- chanics in centrarchid and cichlid fishes: insights into implementation of a slot size of 50 to 85 cm total The participation of the first author in the Fourth Inter- ing effects of introduced Nile Perch (Lates niloticus) key evolutionary innovation’. Journal of Evolution- length for Nile perch (Kizza et. al. 2005). national Conference of the Pan African Fish and Fish- on the foraging ecology of Nile tilapia (Oreochromis ary Biology 9: 641-671. eries Association (PAFFA) in Addis Ababa, Ethiopia niloticus)’. Ecology of Freshwater Fish 15: 470-481. Garrod, D.J. 1960. ‘The fisheries of Lake Victoria, 1954- CONCLUSION (22-26 September 2008) was financially supported by Cadwalladr, D.D., 1965. ‘The decline in Labeo Victorianus 1959’. East African Agricultural and Forestry Jour- Dramatic changes occurred in the ecosystem and fish PAFFA and the Leids Universiteits-Fonds (LUF). Boulenger (Pisces: Cyprinidae) fishery of Lake Victo- nal 26: 42-48 ria and an associated deterioration in some indigenous Getabu, A., Tumwebaze, R., MacLennan, D.N. 2003. fauna of Lake Victoria during the past century as a fishing methods in the Nzoia River, Kenya’. 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